docs: def _retrieve_session(self, session_key):
          """
          :type session_key: SessionKey
          :returns: SimpleSession
          """
          session_id = session_key.session_id
          if (session_id is None):
              msg = ("Unable to resolve session ID from SessionKey [{0}]."
                     "Returning null to indicate a session could not be "
                     "found.".format(session_key))
              logger.debug(msg)
              return None
  
          session = self.session_store.read(session_id)
  
          if (session is None):
              # session ID was provided, meaning one is expected to be found,
              # but we couldn't find one:
              msg2 = "Could not find session with ID [{0}]".format(session_id)
              raise ValueError(msg2)
  
          return session